Support materials for an imaging element often employ layers comprising glassy, hydrophobic polymers such as acrylics, styrenics, and cellulose esters, for example. One typical application is as a backing layer to provide resistance to abrasion, scratching, blocking, and ferrotyping. For coating applications, the glassy polymers are normally dissolved in a solvent at very low solids to ensure low coating solution viscosities for good coatability at high coating speeds. Coating techniques employed include one to three layer extrusion dies (commonly referred to as X-hoppers), air knife, roller coating devices, meyer rods, knife over roll, and so on.
For coating solutions comprising soluble polymers of reasonably high molecular weights, for example, larger than 50,000, the solution viscosity is a strong function of polymer concentration. For example, Elvacite 2041, a methyl methacrylate polymer sold by E. I. DuPont de Nemours and Co., has been described in the photographic art to form scratch protective layers for photographic materials. The polymer is normally dissolved in an organic solvent such as methylene chloride to form a clear solution. At concentrations above, for example, 4 to 5 wt %, the Elvacite 2041 solution viscosity is at least 20 cps at ambient temperature. Such viscosity values are too high for coating applications commonly used in photographic support manufacture, for example, roll coater and skim-pan air-knife coating techniques, which require a coating solution viscosity in the range of from one to a few centipoises. Therefore, photographic manufacturers have to keep the solids below 3 wt % for low solution viscosities and good coatability at high coating speeds.
Polymer solutions of low solids are useful for applications where low dry coating coverages (&lt;500 mg/m.sup.2) can meet the requirements of physical and mechanical properties of an imaging system. More advanced imaging applications need higher dry coating coverages for better physical and mechanical properties. To obtain high dry coating coverages, more coating solution per unit area (wet coverage) has to be applied by using low viscosity/low solids polymer solutions since high viscosity/high solids polymer solutions cannot be coated at low wet coverages at high coating speeds (some coating methods may allow one to coat high viscosity polymer solutions at high wet coverages, but they still suffer from the disadvantages mentioned below). In general, higher wet coverages mean more solvent recovery and higher cost for drying. Furthermore, due to both manufacturing limitations and requirements of an imaging element for other physical and mechanical properties, the wet coverages cannot be increased under certain conditions and for certain applications. For example, high coating wet coverages and the high levels of solvent retained in the film support as a result of these high wet coverages may have a big impact on both dimensional stability and sensitometric properties of an imaging element. One may use resins of low molecular weight to lower the solution viscosity; however, the resultant dry coatings may not have adequate physical and mechanical properties.
Alternative approaches employing low viscosity, dispersed polymer particle-containing coating compositions have been described for paint and automotive coating industries. The use of such compositions in photographic applications has not been disclosed. For example, U.S. Pat. No 4,336,177 describes a solvent coating composition comprising non-aqueous dispersible composite polymer particles larger than 0.1 .mu.m. The particle has a core with a glass transition temperature (Tg) of about 10.degree. C. less than the polymerization reaction temperature. The particles are stabilized by block or grafting copolymers and can be transferred directly from aqueous medium to a non-aqueous medium. U.S. Pat. No. 4,829,127 describes a coating composition comprising composite resin particles. Such particles are prepared by solution polymerization techniques in reaction vessels containing initiator, solvent, polymerizable monomers, and crosslinked particles. U.S. Pat. No 3,929,693 describes a coating composition comprising a solution polymer and polymer particles, where the polymer particles have a crosslinked rubbery core below 60.degree. C. and a grafted shell having molecular weight of 1,000 to 150,000. Reportedly, such coating compositions are more stable toward premature separation and flocculation. U.S. Pat. No. 3,880,796 describes a coating compostion comprising thermosetting polymer particles containing insoluble microgel particles having a particle size of from 1 to 10 .mu.m. U.S. Pat. No 4,147,688 describes a dispersion polymerization process of making crosslinked acrylic polymer microparticles having a particle size of from 0.1 to 10 .mu.m. U.S. Pat. No. 4,025,474 describes a coating composition comprising a hydroxyfunctional oil-modified or oil-free polyester resin, aminoplast resin, and 2 to 50% of crosslinked polymer microparticles (0.1 to 10 .mu.m) made by a dispersion polymerization process. U.S. Pat. No. 4,115,472 describes a polyurethane coating compostion comprising an ungelled hydroxy-containing urethane reaction product and insoluble crosslinked acrylic polymer microparticles (0.1 to 10 .mu.m) made by a dispersion polymerization process. Such coatings are reportedly useful for automotive industries.
There are significant differences in designing coating compositions for photographic applications from those for paint and automotive coating industries. The coating techniques and coating delivery systems are different so that they need different coating rheologies. The drying time in exterior and interior paint and architectural coating applications is on the order of hours and days, and in the automobile industry on the order of 10 to 30 min. However, in the photographic support manufacturing process the drying time for coatings is typically on the order of seconds. Often the drying time for solvent-borne coatings is as brief as 10-30 seconds for high speed coating applications. These differences put additional stringencies on the coating composition for photographic materials. For example, the coating viscosity needs to be on the order of less than 10 cps, and more often less that 5 cps, instead of on the order of one hundred to several thousand cps as in other coating industries. A typical dry coating thickness for photographic materials is on the order of less than 2 .mu.m, and more often less than 1 .mu.m. The film formation and film quality are especially critical. The tolerance on defects caused by polymer gel slugs, gelled particles, dust, and dirt is extemely low. This requires special precautions in delivery processes. The coating solutions need to be very stable toward, for example, high speed filtration and high shear.
Aqueous coating compositions comprising water dispersible polymer particles have been reported to be useful for some photographic applications. For example, they have been used as "priming" or subbing layers on film support to act as adhesion promotion layers for photographic emulsion layers, and used as barrier layers over, for example, a vanadium pentoxide antistatic subbing layer to prevent the loss of antistatic properties after film processing as described in U.S. Pat. No. 5,006,451. While these coating compositions are attractive from environmental considerations, the slow evaporation rate of water coupled with its extremely high heat of vaporization causes drying problems which are either not normally encountered or can be easily overcome in solvent-borne systems. Therefore, for manufacturing processes with conventional organic solvent drying capacity, the use of water-borne coating compositions often leads to very unsatisfactory results. In addition, challenges still exist to develop water-based coatings that provide similar physical and chemical properties in the dried film that can be obtained with organic solvent-based coatings.
Aqueous coating compositions comprising core/shell polymer particles have been disclosed for photographic materials as ferrotyping resistance layers in U.S. Pat. No. 4,497,917, where the polymers are described as having a core with a Tg of greater than 70.degree. C. and a shell with a Tg from 25.degree. to 60.degree. C., and as subbing layers in U.S. Pat. No. 4,977,071 and U.S. Reg. No. H1016, where the polymers are described as a vinylidene chloride copolymer core/shell latex. U.S. Pat. No. 5,366,855, issued Nov. 22, 1994, describes for imaging elements a coalesced layer comprising film-forming colloidal polymer particles and non-film forming colloidal polymer particles. This layer is coated from an aqueous medium and contains polymer particles of both high and low glass transition temperatures. Other aqueous coating compositions that comprise core/shell polymer particles are described in U.S. Pat. Nos. 4,683,269, 4,613,633, 4,567,099, 4,478,974, and 4,134,872. The use of these compositions in photographic films has not been disclosed.
It can be seen that various approaches have been attempted to obtain useful organic solvent-based coating compositions with low viscosity and high percent solids. However, the aforementioned prior art references are deficient with regard to simultaneously satisfying all the physical, chemical, and manufacturing requirements for a solvent-borne coating for more advanced imaging applications. The present invention provides a coating composition which meets all of these requirements while avoiding the problems and limitations of the prior art.